High strength high conductivity aluminum alloy windings in large core form transformers

ABSTRACT

A method is disclosed for forming a coil configuration particularly for use in a large core form transformer. The steps include solution heat treating an age hardenable aluminum base alloy in the form of wire, winding the wire into coil configuration and thereafter age hardening the coil. Alternatively, the solution heat treated wire may be aged to an equivalent of a quarter hard temper and thereafter coiled for use in a core form transformer.

States Patent 1 [191 Kunsman 1 Nov. 19, 1974 HIGH STRENGTH HIGH CONDUCTIVITY ALUMINUM ALLOY WINDINGS IN LARGE CORE FORM TRANSFORMERS [76] Inventor: Laurence D. Kunsman, 113 Echo Related US. Application Data [62] .Division of Ser. No. 168,770, Aug. 3, 1971,

abandoned.

[52] US. Cl l48/l2.7, 29/605, 148/159 [51] Int.- Cl....... C21d 9/52, C22c 21/04, C22f H04 [58] Field of Search 148/l2.7, 159, 32.5, 193; 29/605, 606

[56] References Cited UNITED STATES PATENTS 1,165,558 12/1915 Thordarson 29/605 1,695,044 12/1928 Hallmann l48/l2.7

2,572,562 10/195] Harrington 148/l2.7 X

Duff et a1 29/605 X OTHER PUBLICATlONS Alloy Digest, Al-42, Al-155, Al-194, Aug. 1956, May 1966, Jan. 1970, Engineering Alloy Digest Inc., Upper Montclair, NJ. The Properties of Aluminum and Its Alloys, Aluminum Development Co., London, Dec. 1955, pgs. 142-145.

Primary Examiner--C. Lovell Attorney, Agent, or Firm-R. T. Randig V 57 ABSTRACT A method is disclosed for forming a coil configuration particularly for use in a large core form transformer. The steps include solution heat treating an age hardenable aluminum base alloy in the form of wire, winding the wire into coil configuration and thereafter age hardening the coil. Alternatively, the solution heat treated wire may be aged to an equivalent of a quarter hard temper and thereafter coiled for use in a core form transformer.

1 Claim, No Drawings HIGH STRENGTH HIGH CONDUCTIVITY ALUMINUM ALLOY WINDINGS IN LARGE CORE FORM TRANSFORMERS This is a division, of application Ser. No. 168,770 filed Aug. 3, 197i now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to core form transformers and more particularly to coils for use in core form transformers having a rating in excess of about mva.

2. Description of the Prior Art While the method of the present invention is generally directed to core form transformers, it may also be applicable to shell form transformers. In this respect, it is noted that the difference which resides between a shell form and a core form transformer may be described simply in that in a core form transformer, the coils surround the individual legs of the core material whereas in a shell form transformer the core material surrounds the coils.

The present practice of forming a core form transformer of a rating in excess of about 10 mva has been to employ a core material and to place windings of copper about the individual legs of the core. This results from the fact that the design of the transformer is predicated upon the rating thereof and the amount of copper employed for both the primary and the secondary windings is balanced against the efficiency of thecore material.

Copper has been employed for two reasons, (1) electrical conductivity, and (2) strength. It will be appreciated that, in large transformers, space considerations are essential where the ratings are high. Accordingly, material with high electrical conductivity is utilized so that smaller coils can be employed with the result that more efficient cooling is obtained at a given power rating output. Since copper has the desired high electrical conductivity it eminently meets this criteria. Secondly, copper possesses the desired mechanical properties so that upon the short-circuit operation of the transformer, the peak forces involved in the individual turns or windings of the coil are such that the yield strength of the material will not be exceeded during the normal life span of the operation of the transformer especially at the temperatures involved during such short-circuit operation.

Recently the price of copper has risen to such an extent that designers of large core form transformers are turning their attention to other materials as a suitable substitute for copper. Of primary consideration. among designer's today is the use of electrical conductivity grade of aluminum. Electrical conductivity grade of aluminum normally exhibits between 60 and 62 percent of the electrical conductivity of copper of an equal cross-section and has sufficient ductility that it can be bent edgewise at coil cross-over points during coiling. Moreover, electrical conductivity grade of aluminum has low spring-back which is associated with coiling and as a result better maintains the form of the coil in its wound configuration.

Unfortunately, however, where such electrical conductivity grade of aluminum is employed in core form transformers in excess of about l0 mva, it lacks sufficient resistance to deformation stresses present during short-circuit temperature increases during the opera tion of such transformers that the coils have' become severely distorted, resulting in degradation of the insulation as well as impeding the cooling thereof with the result that transformer life has been materially shortened. Attempts to improve the strength of electrical conductivity type aluminum for use in such coil configurations through work hardening has resulted in an insufficient rise in the yield strength associated with the work hardened material with the result that deformations still occur during short-circuit operation. Moreover, in attempting to sufficiently work harden the material to gain the added strength necessary to resist such deformation, the ductility has been sufficiently impaired that edgewise bending at coil crossover points has become quite difficult and in addition the springback associated with the severely work hardened electrical conductivity aluminum made it unsuitable to winding of the wire material into coil configuration with the proper spacing between the adjacent convolutions of the coil and container tank.

Typically, the copper which has been utilized heretofore in its eighth to quarter-hard tempers, exhibits a yield strength of about 28,000 to 30,000 psi measured at room temperature. However, since the temperature of the coil during short-circuit operation of the transformer can be as high as about 250C, the elevated temperature strength of about 14,000 psi at this temperature during short circuit operation appears tobe adequate for such resistance to deformation as associated with the presently used core form transformers. ln

'order to alleviate the difficulties encountered with the prior art production of core form transformers it has been discovered that a properly age hardenable aluminum alloy can now be employed with satisfactory results.

SUMMARY OF THE lNVENTlON The method of the present invention envisages the forming of a coil for a core form transformer by solution treating an age hardenable aluminum base alloy in the form of wire. Thereafter, the solution heat treated alloy in the form of wire is wound into a predetermined coil configuration and thereafter the coil is age hard- DESCRIPTION OF THE PREFERRED EMBODlMENT In core form transformers having a rating of about 10 mva and greater, high conductivity grade of aluminum, that is a grade of aluminum that has a conductivity of about 62 percent of that of copper has not been employed in such large transformers because such material, regardless of heat treatment or prior work history, does not have sufficient resistance to deformation stresses present during short circuit operation with the associated temperature increases during the operation of such transformers. During such short-circuit operation, a transformer which normally operates at a temperature of about 90 to l C., may'within a time period of less than 1 second, find the temperature abruptly increased to a value of between 150 and 250C. depending upon the power rating of the transformer during such short-circuit operation. While an initial temperature rise will take place before the cireuit breaker opens, followed by another rise after the circuit breaker closes, the time to dissipate such excess heat, which can increase the coil temperature to as much as 250C, may extend for'a period in excess of 1 hour. Moreover, with these large increases in temperature during short circuit operation, there is also associated a force which is dependent upon the rating and the amount of current carried in the coils of the transformer. Where the yield strength is exceeded by the force generated by such short-circuit operations, the coils can be-grossly deformed with the resulting injury to the insulation and eventually a total breakdown of the coil depending on the severity of the damage.

It has been determined empirically that it is necessary for the wire material in the coil of a core form transformer to exhibit a room temperature yield strength such that after a cumulative 30 minutes of total operation at an average temperature of about 225C. the yield strength will not be less than l0,000 psi at such elevated temperature. Certain of the age hardenable aluminum alloys satisfy this criteria insofar as the mechanical properties are concerned and still do not provide an untoward sacrifice in the manifested electrical conductivity of these alloys. Accordingly, the present invention contemplates the use of an aluminum base age hardenable alloy which in its heat treated fully age hardened condition exhibits an electrical conductivity of between 57 and 59 percent of the conductivity of electrical grade copper wire.

More specifically, it has been found that an alloy having a composition within the range between about 0.35 percent and about 0.8 percent magnesium, about 0.3 percent and about 0.7 percent-silicon, up to 0.5 percent maximum iron and the balance essentially aluminum with incidental impurities will, in its properly heat treated condition exhibit between 57 to 59 percent of the electrical conductivity of ETP type copper. The age hardenable aluminum base alloy as described hereinbefore, or any other suitably hardenable aluminum base alloy which exhibits between about 57 and 59 percent conductivity of ETP copper may be employed in the method of the present invention.

The shaped age hardenable aluminum base alloy is initially subjected to a solution heat treatment at a temperature within the range between about 800 and about 950F. for a time period of between about minutes and 1 hour and thereafter rapidly cooled to maintain the age hardening components within the solid solution. Ideally, the age hardenable aluminum base alloy is in the form of wire when subjected to said solution heat treatment, the overall results of which are manifested by imparting to the aluminum alloy wire the characteristics of high ductility, low strength and low spring-back. The wire can take any form, for example, circular, square or rectangular in cross-sectional area, the form of which is not critical to the present invention.

The solution heat treated wire is next wound into a desired coil configuration, such winding and configurations being well known in the art. By having the wire in the solution heat treated condition with its high ductility, low strength and low spring-back, a decided advantage is obtained especially-where, for example, double disc continuous windings are employed in such core form transformers. The bending stresses that are encountered, especially at the coil crossover points, are readily accommodated by the induced high ductility and low spring-back so as to effectively form said coils. The low strength aids in readily forming the desired coil shape with less spring-back.

After the wire has been completely wound into the desired coil configuration, the coil is thereafter subjected to an age hardening heat treatment. By thus age hardening the material in coil configuration a coil set is firmly embodied in the coil so that even during shortcircuit operation the configuration is maintained. Thereafter the age-hardened coil configuration is as sembled into the core form transformer in the manner well known in the art.

The age hardening heat treatment of the coil is performed at a temperature within the range between about 325 and about 375F. for a time period ranging between about8 hours and about 12 hours prior to the assembly of the coil into the transformer. By age hardening the material the strengthproperties of the age hardenable aluminum base alloy are altered so that the alloy will possess a yield strength in excess of that required during short-circuit operation.

In another embodiment of the present invention it may be found desirable to over-age the aluminum base alloy to effect the equivalent of a quarter hard temper thereto in order to obtain the optimum distribution between mechanical and electrical properties. Since the alloy, that is the age hardenable aluminum base alloy, may be aged at a higher temperature or for a longer period of time in order to over-age the same, care must be taken not to under-age since the electrical conduc tivity will be grossly impaired. In this particular embodiment the same age hardenable aluminum base alloy composition recited above is employed, is subjected to solution heat treatment preferably at a temperature within the range between about 800 and about 950F. following which the alloy is rapidly cooled to room temperature in order to preserve the age hardening components in solution within the age hardening aluminum base alloy.

The solution treated aluminum base alloy is thereafter age-hardened to effect a predetermined temper thereto, which temper may be described as not in excess of the equivalent of about one-quarter hard. By preliminarily over-age hardening the aluminum base alloy it is possible to provide an optimum balance between yield strength, ductility, and spring-back on the one hand and electrical conductivity on the other. While the high ductility normally associated with the solution heat treated materialwill be reduced somewhat, nonetheless the alloy wire with an equivalent quarter hard temper will retain sufficient ductility to make even the most difficult crossover bends at the coil end points as presently used in core form transformers. While spring-back will be somewhat greater, nonetheless, the forces employed will be sufficiently high that undue spring-back will not be a problem. The quarter hard temper aluminum base alloy in wire form is thereafter wound into coil configuration and assembled into a core form transformer and will exhibit sufficient strength so as to remain relatively uneffected by the forces which are encountered within the coil during minum in the form of wire for imparting thereto high ductility, low strength and low spring-back, winding the solution heat treated wire into predetermined coil configuration and thereafter age hardening the wire in coil configuration at a temperature within the range between 325F and 375F for a time period ranging between about 8 and 12 hours, to provide a yield strength level of at least 10,000 psi during short-circuit operation conditions of the transformer. 

1. IN THE METHOD OF FROMING A COIL FOR A CORE FORM TRANSFORMER THE STEPS COMPRISING, SOLUTION HEAT TREATING AT A TEMPERATURE WITHIN THE RANGE BETWEEN 800* AND 950*F AN AGE HARDENABLE ALUMINUM BASE ALLOY HAVING A COMPOSITION WITHIN THE RANGE BETWEEN 0.35 PERCENT TO 0.8 PERCENT MAGNESIUM, 0.3 PERCENT TO 0.7 PERCENT SILICON, 0.5 PERCENT MAXIMUM IRON AND THE BALANCE ALUMINUM IN THE FORM OF WIRE FOR IMPARTING THERETO HIGH DUCTILITY, LOW STRENGTH AND LOW SPRING-BACK, WINDING THE SOLUTION HEAT TREATED WIRE INTO PREDETERMINED COIL CONFIGURATION ND THEREAFTER AGE HARDENING THE WIRE IN COIL CONFIGURATION AT A TEMPERATURE WITHIN THE RANGE BETWEEN 325*F AND 375*F FOR A TIME PERIOD RANGING BETWEEN ABOUT 8 AND 12 HOURS, TO PROVIDE A YIELD STRENGTH LEVEL OF AT LEAST 10,000 PSI DURING SHORT-CIRCUIT OPERATION CONDITIONS OF THE TRANSFORMER. 